<?php
use  yii\helpers\Html;
use yii\widgets\ActiveForm;

$form = ActiveForm::begin([
    'id' => 'news',
    'options' => ['class' => 'form-horizontal'],
])

?>
<?=$form->field($model,'username')?>
<?=$form->field($model,'password')->passwordInput()?>

    <div class="form-group">
        <div class="col-lg-offset-1 col-lg-11">
            <?=Html::submitButton('Login',['class'=>'btn btn-primary'])?>
        </div>
    </div>
<?php  ActiveForm::end();?>